In the oil and gas industries it is often desirable to drill a number of wells from a single surface location. Each well extends downwardly and outwardly from the surface location in a different direction from the other wells, i.e. in various different combinations of compass bearing, inclination from vertical, and depth below surface. The initial portion of the well (the section nearest the surface) may be vertical or inclined to the vertical. The direction of drilling frequently requires to be changed, to increase or decrease the inclination of the well, or to correct for an unintended change in direction such as may be caused by geological conditions. Such techniques are generally known as directional drilling.
Directional drilling is especially useful in offshore production of oil and gas since the procedure enables a large area to be drilled from a single platform. However, directional drilling requires effective and efficient control of direction.
In directional drilling, control of direction may be undertaken by downhole stabilisers. In the most general terms, known forms of downhole stabilisers consist of a collar-like device or asembly fitting around or into a drill string near the lower end of the string. A fixed stabiliser will centralise the drill string and tend to straighten the well being drilled. Replacement of such a fixed stabiliser with one of lesser diameter will allow the drill string to bow, and thus sustain or increase curvature of the well being drilled. Upon achievement of the desired direction after curved drilling, a maximum diameter stabiliser will normally tend to maintain the desired direction by straight drilling.
With each change in diameter of a fixed stabiliser, the entire drill string must be withdrawn to have the stabiliser removed from the string and replaced by another stabiliser of the requisite different diameter, followed by reinsertion of the string down the well. With the intention of avoiding such unproductive activity, proposals have been made for downhole stabilisers whose effective diameter can be varied under remote control such that a stabiliser near the bottom of a well can have its diameter adjusted under control from the surface location and without being raised to the top of the well. However, such adjustable stabilisers have either required hydraulic systems, or excessively complicated mechanical systems. In the mechanical systems, either an actuating link was needed from the surface location to the stabiliser, or continuous control of the downward force on the drill string was needed to effect stabiliser diameter changes. In particular, previous designs of downhole stabiliser not requiring a downhole link additional to normal drill string components did not also have the ability to operate with a full range of normal drilling forces independently of diameter-changing operation of the stabiliser.
It is therefore an object of the invention to provide a downhole stabiliser which obviates or mitigates these disadvantages.